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Voracious Wurm enters the battlefield with X +1/+1 counters on it, where X is the amount of life you've gained this turn.

Seeing as this is not a triggered ability, but a replacement effect, would I be able to gain life at my end step with Wall of Reverence's triggered ability, then add to the stack my Wurm's ability to gain X +1/+1 counters? (After all, that's life I gained in my turn.)

The following rules seem relevant here:

603.3b If multiple abilities have triggered since the last time a player received priority, each player, in APNAP order, puts triggered abilities he or she controls on the stack in any order he or she chooses. (See rule 101.4.) Then the game once again checks for and resolves state-based actions until none are performed, then abilities that triggered during this process go on the stack. This process repeats until no new state-based actions are performed and no abilities trigger. Then the appropriate player gets priority.

614.1. Some continuous effects are replacement effects. Like prevention effects (see rule 615), replacement effects apply continuously as events happen -- they aren't locked in ahead of time. Such effects watch for a particular event that would happen and completely or partially replace that event with a different event. They act like "shields" around whatever they're affecting.

1 Answer
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No. Replacement effects do not use the stack; as rule 614.1 (which you quoted) says, "[R]eplacement effects apply continuously as events happen" — i.e., they happen immediately, at the same time as the event being replaced/modified (in this case, Voracious Wurm entering the battlefield). Thus, if Voracious Wurm enters the battlefield before the beginning of the end step, it won't benefit from Wall of Reverence's lifegain (but the Wall will be able to target the Wurm later).

However, if the Wurm somehow (say, by giving it flash) enters the battlefield during the end step (or, less likely, the cleanup step) after the Wall's ability resolves, its P/T will be boosted by the lifegain from the Wall — but, of course, the Wall can't target the Wurm until your next end step.

And if, for some strange reason, the Wurm enters the battlefield while the Wall's ability is still on the stack, the replacement effect will happen before the Wall's ability resolves, so the counters won't be boosted, and as targets for the Wall's ability must have already been chosen by that point, the Wall won't be able to target the Wurm that turn either.